Dimples on receiver and bolt

[Patrick Chadwick]

This may sound like a dumb question, but what's the difference between "High Pressure Testing" and Hardness testing.

High pressure testing applies a force over a considerable area, but remains within the elastic limits of the material. A proof round, for instance, is not supposed to permanently distort the chamber! But the old CUP measurement was derived from the measured distortion of the copper block. The intention is to check that the material under test will indeed not be stressed beyond the elastic limit in the intended appplication.

Hardness testing measures just what that elastic limit is, by applying a much higher force than a pressure test, but to a very small area that is indeed distorted beyond the elastic limit, and the hardness figure is derived from measurement of that distortion. If the pressure produced on the tiny spot of a Rockwell tester were to be applied all over a chamber, the chamber would not just distort, it would explode.